adding a couple passive increases to the market place with an increasing crown cost could be a neat way to reward more active players, and give players that warm fuzzy grinded/farmed for hours to get maxed out feeling.

small bonuses like -

1% increase to base health for all units
1% increase to base attack for all units
1 point of regen base for all units that doesn't go toward the cap
2 point increase to base movement for all units
5000 more storage space in settlment
etc
etc
etc

now have these bonus cost an exponential amount of crowns and and cap at something that makes sense.

maybe something like max 5 upgrades for each bonus

1% increase to base health for all units (Up to 5%)
1% increase to base attack for all units (Up to 5%)
1 points of regen base for all units that doesn't go toward the cap (Up to 5 points regen)
2 points increase to base movement for all units (Up to 10 movement)
5000 more storage space in settlment (Up to 25000 total extra space)

In my opinion this would be a great way to get players exited about playing more, or paying more. while not being significant enough of an advantage to alienate more casual or newer players from pvp against players who have bought all the upgrades.

Hope this isn't a rip of someone elses suggestion, I didn't read any of them!